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common rail


  • Operation
  • Differences between the conventional injection system and common rail
  • Low pressure section
  • High pressure section
  • Measuring voltage and current on the electromagnetic injector
  • Engine electronics

Common rail is an injection system that has been used on diesel engines since 1997. The injectors are controlled by the engine control unit. Both the opening and closing of the injector (the injection time) and the number of injections per combustion cycle are determined by the engine control unit. The engine control unit calculates the injection time based on a number of factors, such as speed, load, outside air and engine temperature, etc.
The high pressure pump supplies the fuel pressure to the fuel gallery. There is always a constant pressure in the fuel gallery. All injectors are connected directly to the fuel gallery. The fuel pressure is therefore also directly on the supply line of each injector. Only when the atomizer receives an opening signal from the engine control unit will it open. The pressure from the fuel gallery will now enter the cylinder through the injector. The injection stops as soon as the engine control unit terminates the signal.

The green line shows the low pressure fuel supply line.
The electronic feed pump (11) pumps the fuel with a pressure of max. 5 bar via the filter element (9) to the high-pressure pump (1). A high pressure line (red) runs from the high pressure pump to the fuel rail. There is a fuel pressure in the fuel rail that depends on the speed of the high-pressure pump. The rail pressure sensor registers this value and constantly transmits the current fuel pressure to the engine control unit.
The high pressure lines of all injectors are connected to the fuel rail, as can be seen with the number 8 fuel rail and the number 16 injector. The return line (blue) returns all excess fuel from the injector, fuel rail and high pressure pump back to the tank. There is a constant circulation of fuel to cool the components, which are often located in the engine compartment.

Differences between the conventional injection system and common rail:
For (conventional) diesel engines without common rail injection (i.e. with a high pressure line pump, rotary distributor pump or an electronically controlled distributor pump) the injectors are opened by the pressure of the fuel itself.
The fuel pump rotates with the speed of the camshaft and builds up pressure at the right time. The pressure build-up and the injection therefore depend on the timing of the fuel pump relative to the camshaft. Therefore, when replacing the timing belt, the fuel pump must always be blocked as well.

In common rail engines, the fuel is injected when the engine control unit gives a signal. In the first generation of common-rail engines, the position of the pump therefore did not matter. It could be turned into any position when installing the timing belt. The pump supplies a constant fuel pressure to the injector rail.
Nowadays all engines are tuned much more precisely. The pump now also often has to be blocked. This is to prevent vibrations related to the pressure build-up of the pump. The pumps are now constructed in such a way that the peaks of the pressure build-up occur at the same time as the compression stroke of the engine. The engine runs more smoothly and the timing belt is less heavily loaded.

Low pressure section:
The low pressure section includes the fuel tank, electric booster pump, fuel filter, low pressure fuel line and return line. These components are described below.

  • Fuel Tank: This is where the fuel is stored. The tank capacity can vary between 30 and 70 liters for lighter and heavier luxury passenger cars. Click here for more information about the fuel tank.
  • Electric lift pump: mounted in the tank. This pump ensures that the fuel is pumped from the tank under low pressure to the high pressure pump (in the engine compartment). Common rail diesel engines do not always have an electronic booster pump. Sometimes a gear pump is built into the high pressure pump. From the high-pressure pump, both the fuel is sucked out of the tank and pressure is built up towards the fuel rail. Click here for more information about the booster pump.
  • Fuel filter: Fuel can contain contaminated particles. These particles remain in the filter material, so that they cannot enter the injection system. The fuel filter also serves as a water separator. Diesel fuel also contains moisture. This moisture is very bad for the pump and the injectors / pipes. This can cause corrosion on the inside of the components. To prevent this, the water is also separated from the fuel and remains in the filter. This filter must be drained or drained periodically. to replace. 
  • Low pressure fuel line: This fuel line runs from the electronic booster pump to the high pressure pump. The pressure on this line is approximately 5 bar.
  • Fuel return line: The fuel that is pumped too much is returned to the tank via the return line. The return fuel also serves for cooling, as it dissipates the heat. Return fuel must therefore always be available. At the moment of deceleration (the engine is braking), no fuel is injected into the combustion chamber. The amount of return fuel is greatest at that time.
    The return fuel can also be used to identify whether an injector is left open unintentionally. This can be caused, for example, by contamination or a defect in the injector, or an error in the control unit of the engine control unit. By disconnecting the return lines of all injectors and collecting them at the same time, the mutual difference can be viewed. If 1 injector has a noticeable little return fuel, it could very well be that the injector remains open too long. Too much fuel is being injected. This can be seen in the image below. Here one injector has no return fuel.
Fuel return control

High pressure section:
The high pressure section includes the high pressure pump, fuel gallery, high pressure fuel lines and injectors.

  • high pressure pump
    The high-pressure pump is designed as a plunger pump and ensures that a fuel pressure in the fuel gallery (depending on the system) remains at a constant pressure. This is 1300 bar with the first generation common rail engines (from 1997) to 2000 bar with the current systems. The higher the injection pressure, the smaller the fuel droplets and the better the combustion and thus the exhaust gas emissions. The amount of fuel that the pump supplies to the fuel gallery is limited as the engine requires less. The pressure then remains approximately the same. By controlling an electromagnetic overflow, a control piston moves further and further as a result of spring tension. The rail pressure then decreases. The High-pressure fuel pump page explains in detail the operation of several types of high-pressure pumps, including those of the common rail diesel.
  • Fuel Gallery
    From the high pressure pump, the fuel is pumped to the fuel gallery. A constant fuel pressure prevails in the fuel gallery. Fuel lines run from the fuel gallery to the injectors. The rail pressure sensor is also connected to the fuel gallery (if the rail pressure is too high, the engine management will ensure that the pressure relief valve opens) and there is a return line.
  • High Pressure Fuel Lines
    Because high-pressure fuel lines have to endure high pressures, they must be of strong construction. They are made of metal and are connected with union nuts to both the pump and the injectors. These high pressure fuel lines carry the fuel from the high pressure pump to the fuel rail and from the fuel rail to the injectors. The lines between the fuel rail and the injectors are all the same length and thickness. This prevents mutual injection differences. If the distance between the fuel gallery and cylinder 1 is greater than between the gallery and cylinder 4, a bend is made in the pipe of cylinder 4. Due to this bend, the distance that the fuel from cylinder 4 has to travel becomes the same as that from cylinder 1.
  • Atomizer
    There are electromagnetic or piezo injectors applied. With these injectors, the injection quantity, the injection course and the injection moment can be controlled. There is a constant fuel pressure at the inlet of the injector. This is the same pressure as is present in the fuel rail. This pressure also prevails in the control chamber as long as the solenoid valve is closed. The solenoid valve is controlled by the ECU.
    As soon as the solenoid valve is actuated by the engine management, the injector needle is lifted and the injector injects a certain amount of fuel. Because the rail pressure and the openings of the injectors are always constant, the engine management knows exactly how much fuel is injected in a certain time. Since a minimal deviation always occurs after manufacture, this deviation must be communicated to the engine control unit. After manufacture, the injector is tested. A code is determined with the results of, among other things, the opening pressure and injector quantity. This code is engraved on the injector and can be read by the technician (see image below, the code is 574-221). This way of teaching is the same for both a petrol engine and a diesel engine.
  • Measuring voltage and current on the electromagnetic injector:
    The voltage and current course across the electromagnetic injector can be measured with the aid of the oscilloscope. This can be used to determine whether the injector is correctly controlled by the ECU.
    In the scope image below, the red line is the voltage curve and the blue line is the current curve. The scope image above shows two injections. The left is the pre-injection and the right is the main injection. Other engines can even have three consecutive injections.
    The injector opens at high voltage and current. The voltage is about 80 volts. This high voltage can be achieved thanks to a capacitor in the ECU. This high voltage in combination with a low coil resistance ensures a quick response of the injector. The injector therefore has a short switch-on and switch-off delay. Because the current through the coil generates a lot of heat, it must be limited. Without current limitation, the actual current would be as high as 300 amps. However, that value will never be reached because then the injector coil will have burned out for a long time. 
    The current limitation is indicated by voltage that is constantly switched on and off, between 4,6 and 5,1 ms. During this current limitation, the voltage (12 volts) and the current (12 amps) are still high enough to keep the injector needle open.
    At 5,1 ms the actuation is stopped and the injector needle will close. 
Voltage and current curve

Engine electronics:
The engine management (ECU) calculated from data from sensors (accelerator pedal position sensor, engine temperature, driving speed, crankshaft speed, air volume (air mass meter), intake air temperature, exhaust gas quality (NOx) the amount of fuel to be injected and the time at which it should be injected. It is a tough job to control the injectors. In order to be able to supply a current of more than 300 Ampere in a short time (max. 20 milliseconds) a voltage of up to 80 Volt is necessary.
This is achieved with the charge of capacitors and power amplifier stages.

Engine Control Unit